Speakers convert electrical signals to sound via the use of drivers. A conventional driver 10 for a speaker is shown in FIGS. 1, 2, and 13. Common drivers 10 include a basket or frame to which is attached a lightweight diaphragm 22. Attached to the diaphragm 22 is a wire coil 20 wound round a pole piece 12 that sits at the center of a magnet ring 14 sandwiched between a top plate 16 and a back plate 18.
The magnet ring 14 is usually polarized in the direction of the ring's thickness, e.g., with the north pole 80 adjacent the top plate 16 and the south pole 90 adjacent the back plate 18. As shown in FIG. 3, the magnet ring 14 generates a magnetic flux 30 that is dense in the gap 24 between the top of the pole piece 12 and the top plate 16. The wire coil 20 is located in this gap 24 between the pole piece 12 and the magnet layer 14 and metal plates 16, 18. An electrical signal is applied to the wire coil 20, which in turn makes the wire coil 20 an electromagnet and produces a magnetic field. The wire coil's magnetic field interacts with the magnet and metallic layers' field so as to generate a mechanical force that causes the wire coil 20 to move up or down, depending on the signal sent to the wire coil 20. The motion of the wire coil 20 causes the diaphragm 22 to move or vibrate, which vibration produces audible sounds.
The conventional driver 10 has its short comings. For example, conventional drivers 10 include a back surface that generally is integrated with the back plate 18 of the magnetic circuit. When the diaphragm 22 moves, the air and sound waves 70 formed behind the diaphragm 22 are trapped by the back surface 18, as shown in FIG. 13. This causes distortion of the sound and inhibits the free motion of the diaphragm 22. Further, speakers with conventional drivers 10 are generally thick, e.g., on the order of two to three inches. This makes these speakers less conducive to use in environments in which thinner speakers would be more desirable, such as when mounting speakers in walls or in automobiles.
Another large problem with conventional drivers 10 is the high level of inductance due to the use of the ferrous metal pole piece 12. Any wire coil 20 with a current will create inductance, and the placement of a larger metal pole piece 12 inside the coil 20 creates an even larger amount of inductance. In the speaker, the higher the level of inductance, the greater the distortion in the sound quality. At higher frequencies, inductance is even more a problem.
The use of the ferrous metal pole piece 12 also leads to distortions in the loudspeaker's transducer for other reasons. For example, using a pole piece 12 causes dynamic non-linearity in a moving coil transducer, such as that of the wire coil 20 when in operation. Additionally, movement of the wire coil 20 along the pole piece 12 causes eddy currents and flux modulation. The eddy currents produce heat, which alters the resistance of the wire coil 20. Accordingly, the inclusion of a ferrous metal pole piece 12 in the conventional driver 10 leads to much undesirable distortion in the produced sound.